Airplane control device



Nov. 20, 1951 w. R. SEARS 2,575,532

AIRPLANE CONTROL DEVICE Filed Jan. 20, 1945 4 Sheets-Sheet 1 Nov. 20,1951 w. RQSEARS 5 5 AIRPLANE CONTROL DEVICE Filed Jan. 20, 1945 4SheetsSheet 2 JNVENTOR. W/zL/AM R. SEARS 20, 1951 w. R. SEARS AIRPLANECONTROL DEVICE 4 Sheets-Sheet 3 Filed Jan. 20, 1945 INVENTOR. W'/ LL/AM2. 55,425

Nov. 20, 1951 w, SEARS I 2,575,532

AIRPLANE CONTROL DEVICE Filed Jan. 20, 1945 4 Sheets-Sheet 4 IN VEN TOR.

Patented Nov. 20, 1951 AIRPLANE CONTROL DEVICE William R. Sears,Inglewood, Calm, assignor to Northrop Aircraft, Inc., Hawthorne, Calif.,a corporation of California 7 Application January 20, 1945, Serial No.573,684

11 Claims.

This invention relates generally to aircraft, and more particularly tomeans for influencing the behavior of the airplane control surfaces. I

In'one aspect, the present invention may be characterized as dealingwith and providing a novel and particularly effective solution for theproblem of maintenance of stick-free longitudinal stability in anairplane of either conventional or tailless types. As is well known, athigh angles of attack approaching the stall, and with the onset ofseparation of the boundary layer from the surface of the airfoil, thereis a sudden marked change in the chordwise pressure distribution overthe airfoil, creating an abruptly increased increment of hinge moment inthe control surfaces in the trailing edge of the airfoildisproportionate to the increment of angle of attack. The efiect of thisdisproportionate increase in hinge moment during the last few degrees ofchange in the angle of attack as the wing approaches and passes thestalling angle is to cause an abrupt transition from a relatively slightupwardly trailing tendency on the part of the control surface at low ormoderate angles of attack, to a greatly increased upwardly trailingtendency at only a few degrees more angle of attack. In the case ofelevators or elevons which are rigged to move up or down simultaneously,this increment of hinge moment is transmitted back to the control stickand acts to displace the latter rearwardly from its neutral centeredposition. This rearward displacement of the control stick must beresisted by applying a corrective force to return it to neutral in orderto maintain the equilibrium, or trim, of the airplane, as otherwise thestalling moment produced by the upwardly deflected elevators will causethe airplane to nose up still further to a stall, with consequent lossof control. The requirement of a force on the control stick to return itto neutral from a deflected position is the reverse of the conditionexisting at only slightly lower angles of attack, hence the phenomenonis known as stick-force reversal, and its occurrence during the criticalperiod of take off and landing when the airplane is at a high angle ofattack is highly undesirable. Various expedients have been devised inthe past to counteract this destabilizing effect caused by the trailingtendency of the elevators, but none of these, to my knowledge, has beenentirely effective for all angles of attack.

One of the primary objects of the present invention, therefore, is toprovide a novel and improved means for maintaining the longitudinalstability of an airplane at all angles of attack.

(Cl. 244-82) i Another object of the invention is to providemeans forovercoming or counteracting the increment of hinge moment produced-inthe control surfaces at high angles of attack by reason of the change inchordwise pressure distribution over the wing at or near the stalledcondition. This will eliminate the phenomenon known as aileron snatchingin ailerons located conventionally in the trailing edge of a wing. Thisphenomenon manifests itself as a sudden force reversal on the controlcolumn in a lateral direction, causing the control column to be pulledforcibly over to one side to its extreme position unless restrained bythe pilot, and commonly occurs as the stalling angle is approached,especially if the aileron linkage incorporates a difierential motionarrangement, so that the mechanical advantages of the upand down-goingailerons are-not equal. It results from the up-floating tendency of theailerons near the stall.

A further object of the invention is to provide a system for controlsurfaces which provides both an aerodynamic boost effect augmenting thepilots control force, and a stabilizing efiect tending to resist orovercome the upwardly trailing tendency of the control surface at highangles of attack.

A second aspect of my invention deals with the problem of stabilizing atailless airplane wherein the center of gravity of the airplane islocated aft of the aerodynamic center of the wing. It has long been heldthat inherent longitudinal stability could be had in a tailless airplaneonly by locating the center of gravity ahead of the aerodynamic centerso that the pitching moment of the wing, without control-surfacedeflection, at all angles of attack tends to restore the wing to acondition of equilibrium at which the moment coefficient about thecenter of gravity is equal to zero. In this conventional design oftailless airplanes, the elevators, or elevons, must be defiectedupwardly in order to increase the angle of attack, as when landing ortaking off, but such upward deflection of elevators located in thetrailing edge of and forming a part of the wing imposes a considerabledownload on the wing, neutralizing much of the added lift gained byincreasing the angle of attack. By locating the center of gravity of theairplane to the rear of the aerodynamic center of the wing, and applyinga corrective wing moment through the use of a.

negative or downwardly-deflected elevator, as taught in my pendingapplication, Serial No. 459,592, filed September 24, 1942, now PatentNo. 2,416,958, dated March 4, 1947, a condition of equilibrium isobtained, together with an increased maximum lift coefncient for thewing.

Accordingly, it is another object of the present invention to providenovel and effective means for causing the elevator to deflect downwardlyat high angles of attack to stabilize a tailless airplane of the typedescribed and to increase the maximum lift: coeficcient obtained.

A third aspect of my invention deals with its use in conventionalairplanes to increase the stick-free longitudinal stability of suchairplanes and make them more difficult to stall. Another object of theinvention, therefore, is to provide means responsive to a stalledorpartially stalled condition of the wing for deflecting the elevatorsdownwardly to produce a diving moment in the airplane, reducingthe'angle of attack of the wing and thereby averting the impendingstall.

The above. objects are attained by the use, of a small, freely floatingauxiliary flap which I prefer to call a dipper, locatedin thetrailingedge of the wing and operatively connected, to a tab on its associated.control surface. The flipper is free to trail in accordance with thechordwise pressure distribution over the. win surfaces ahead of the,flipper, unrestrained by the pilot, causing the tab to be deflectedaccordingly to produce the desiredbehavior in the control surface. Bysuitable design of the linkage connecting the flipper to the tab and theproper proportioning of the various elements in the system, the presentdevice can be madeto change both the trailing tendency and the hingemoment of the control surface.

These and other objects and advantages of the present invention willbecome apparent to those skilled in the art after consideration of thefollowing, detaileddescription of the preferred embodiment thereof,reference being had to the ac companyingdrawings, in which:

Fig. l is afragmentary perspective view, partially broken. away, of aportion of an airplane wing showing one embodiment of the presentnventio 2 isa op. pl n tially cut away;

.Fig. 3is an enlarged sectional view taken along the line. 3 -3 in Fig.2 and showing an end view of the control surface;-

FigA is anenlarged-sectional view taken along the line-.4-4 in Fig.2 andshowing an end view of e i per;

Fig, 5. is an enlarged sectional View taken along theline 5- -5-in- Fig.2, showing a portion of the linkageconnecting the flipper to the tab;

, Fig, 6 is a perspective sectional view of another modification of theinvention;

Fig. 'l is an enlarged section taken along the line 1-4 in Fig. 6; V

Fig. 8 is a phantom perspective view of a conventional airplaneembodying one form of the invention; and I Fig. 9-is an enlarged planview of the tail of the airplane, showing more clearly the arrangementof the linkage connectingthe flipper to the tab on the elevator. 7

Figs. 1 to 5, inclusive, of the drawings illustrate my inventionembodied in the control system of a tailless airplane wherein both pitchcontrol and roll control are obtained by control surfaces known aselevons which are rigged for both simultaneous motion in the samedirection to act as elevators and for motion in opposite directions to,act as ailerons. In these drawings, the reference numeral 6 indicates awing of the usual airview of the same, also parfoil shape having anupper skin I and a lower skin 8, which are fixed to the top and bottomedges, respectively, of a spanwise extending spar 9. Located in thetrailing edge of the wing and pivotaily supported thereon for swingingmovement about a transverse or spanwise axis is an elevon iii, shaped tocarry out the airfoil contour of the wing when. in neutral positionandoperatively connected to the control system of the airplane wherebyit can be deflected upwardly or downwardly to produce the desiredcontrol force on the wing.

The elevon H] is preferably, although not necessarily, of the internalbalance type having a nose H which extends. forwardly into the spacebetween the top and bottom skins l, S of the wing, and its structure ismore or less conventional, comprising top and bottom skins l2 and I3attached to a spanwise extending spar M.

Fixed to the front end of the balance nose Ii and extending the fulllength of the elevon Ill. is a flexible curtain seal 15, which is.attachedatilfi to an arcuate wall I? mounted in the wing between the topand bottom skins l and 8, andwhich divides the chamber in the rear edgeof the wing into two separate compartments I18 and i9. Sum..- cientslack is provided in the curtain seal I5 to permit the eleven to beswung to its extreme raised or lowered position.

The elevon it is curved or contoured chordwise thereof on its upper andlower surfaces at 2 l. and 22 t9 form arcs'of a circle having the hingeaxis of theelevon as its center. These curved portions 2!, 22 are spaceda slight distance. from the adjacent trailing edges of the upper andlower skins of the wing so as to provide slots 23 and 24 which arecontinuously open and thus provide constant communication between thechambers it and i9 and the pressure conditions at the upper and lowersurfaces, respectively, of. the wing and elevon. By virtue of thecurvature at 2| and 22, the slots. 23 and 2d are maintained continuouslyand uniformly spaced from thetrailing; edges of the upper and lower wingsurfaces for all posi-. tions of the elevon.

When the airplane is inflight, there is. normally greater air pressureat theunder. surface. of the wing than at the upper surface thereof, andinasmuch as the upper and lower chambers i8, i9, respectively, are inconstant communication with the pressure conditions at the. upper andlower sides of the wing through the slots 23 and 25, the same pressuredifferential prevails on opposite sidesof the nose H andv curtain I5.Deflection of the elevons in one direction. or the other causes a changein the pressure conditions at the slots 23, 24, altering the pressuredifferential on opposite sides of the balance nose i I and producing ahinge moment in the direction to assist the pilot in deflecting theelevon. The above-described internal balance construction and itsmannerof operation are well known in the art, and further descriptionthereof is not deemed necessary. It will be understood that anyotherform of aerodynamic balance might be used in place of the constructiondescribed, or such balances might be dispensed with entirely if theiruse is not required for a particular airplane design.

A plurality of spaced bearings carried by suitable brackets mounted onthe wing provide; support for the elevons,.one of such bearings beinglocated at the end of the elevon and indicatedby; the reference numeral28. Bearing28 differs from: the others (not shown) in that it has ahollow center to permit passage of a push-pull rod 29, the purpose ofwhich will be described presently. Space for the bearing 28 is providedby a cutout 36 formed in the elevon ahead of the spar l4 at one endthereof, and a T-shaped bracket member 3l.is arranged in such cutout andfixed to the front side of the spar 14 with its flange 32 extendingforwardly. The flange 32 is apertured to receive a tubular bearing 33(see Fig. 5), and is rotatable thereon, said bearing being journaled, inturn, in a bearing plate 34 held between a pair of parallel brackets 35which are mounted on and extend rearwardly from the rear spar 9 of thewing. A washer 36 is arrangedbetween members 32 and 34 to space themapart, and the assembly is secured by a nut 40 threaded onto the end ofthe tube. The other end of the tube is enlarged in diameter at 41 toform a shoulder confining the plate 34, and is provided with a flange42.

Hinged to the trailing edge of the elevon i is a tab 45 which issupported on bearings 46 mounted on a wall 48 carried by the elevon. Abracket or horn 52 is fixed to the tab at one of the hinge pivotsthereof and extends downwardly, said horn being provided with a seriesof vertically spaced holes through which a pin 52 is passed to connect apush rod or link 53 with the horn. The function of the tab 45 is toimpose a hinge moment on the elevon tending to swing the latter upwardlyor downwardly, depending upon the direction of displacement of the tab.Arranged alongside the elevon 15 immediately adjacent one end thereof isan element 56 which I call a flipper, which is connected through linkageto be described presently with the tab 45 whereby the position of thelatter relative to the elevon I0 is determined by the position of theflipper relative to the wing. The flipper 56 is attached to the frontends of the plates 6!] and 5! to provide static balance for the flipper.The forward portions of the skins 51, 58 are arcuately curved about thehinge axis of the flipper as a center to clear the trailing edges of thesurfaces of the wing and are interrupted to form a gap at 'l I. Theright-hand end plate BI is bolted or otherwise suitably attached to theflange 42 of the bearing Q8, while the left-hand end plate 66 issupported on the wing structure by means of a bearing 63' coaxial withbearing 28.

A link 64 is connected at its rear end by a pivot bolt 65 to the leftwall 60 of the flipper at a point spaced vertically above the axis ofbearing 63,

and extends forwardly therefrom. A yoke 65 at the front end of link 64is pivotally connected to the outer end of a spanwise extending arm 61of a bell crank member 68, arm 61 being welded to a vertical tube 69which is arranged between the end plates 66, 6| and is journaled at itstop and bottom ends in bearings fixedly mounted on upper and lower skinsI, 8 of the wing. Fixed to the tube 69 below arm 61 is an arm 14extending rearwardly therefrom through the gap H in the leading edge ofthe flipper and connected at its end to a yoke carried at the left-handend of push-pull rod 29. p

- Mounted on the right-hand end of the rod 29 is a yoke 80, which isconnected to the forwardly extending arm 8| of a bell crank 82. The bellcrank 82 is pivotally supported by a bracket .83

mounted on the back side of spar l4, arm 8|. extending through asuitable slot in the spar. The

other arm.84 extends laterally and is connected at its outer end to thefront end of link 53.

, The flipper 56, being pivotally supported on the wingand free tofloat, tends to trail in accordance with the chordwise pressuredistribution over the wing surfaces aheadof the flipper at,

all times.v Thus, at high angles of attack when the wing is stalled orpartially stalled, the flipper tends to trail up to the positionindicated at 56a in Fig. 4. This relative motion of the flipper istransmitted to the tab throu h the agency of the connecting linkage,deflecting the tabto the position indicated at 45a in Fig. 3. The hingemoment onthe elevon produced bythe upwardly deflected tab acts tomaintain the elevon in its neutral position, preventing the latter fromswinging upwardly with the flipper and producing a stalling moment onthe airplane. The effectiveness of the flipper 56 can be varied bymoving the connection of the link 53 with horn to one of the other holes5|.

If the airplane should encounter a sudden downward gust so that theangle of attack is momentarily negative, the flipper 56 will swingdownwardly, causing the tab 45 to be deflected downwardly. By properdesign of the flipper and connecting linkage, the tab 45 can be made toproducev any desiredeffect on the behavior of the elevon l0. Thus, theflipper can be made to. eliminate the force reversal heretoforeexperienced in tailless airplanes at high angles of attack when the wingis stalled or partially stalled or even produce a negatively trailingelevon for increasing the maximum lift coeflicient of the wing in thesamemanner as a flap.

The invention illustrated in Figs. 1 to 5, inclusive, and described upto this point, is effective to produce deflection of the tab responsiveonly to displacement of the flipper relative to the wing 6. Undercertain conditions, it is also desirable to obtain deflection of the tabresponsive to displacement of the elevon relative to the wing, in whichcase the tab acts as a boost tab imposing a hinge moment on the elevonto aidthe pilot in swinging the elevon upwardly or downwardly.

Accordingly, another embodiment of my invention is illustrated in Figs.6 and 7 in which deflection of the tab is effected by displacement ofeither the flipper or the elevon relative to the wing. In Figs. 6 and 7,the elevon l6 and the flipper 56 are shown more or less diagrammaticallyarranged alongside one another and supported on the wing for independentvertical swinging movement about a common hinge axis 90. The internalbalance arrangement described in the previous embodiment has beenomitted in this construction, and the leading edge of the elevon isshaped in a cylindrical curve at 9i having the hinge axis 96 as itscenter. Similarly, the counterbalance for the flipper 56 has beenomitted, and the leading edge of the flipper is likewise curvedcylindrically at 92 about the hinge axis 96 as its center. It should bekept in mind, however, that the arrangement shown is merelydiagrammatical to illustrate the principles of the invention, and thatsuch elements as balances and counterweights may be employed in any oftheir well. known forms according to the dictates of the particularinstallation without departing from thesubstance of the invention.

;a'rmsi9'I whichiareconnectedzbynables8to horns i 1'99 ion the tab. 1Thecables 98 ientersltheiielevons :10. throughalverticalzslot J sin'rtheleadingedge portion 9 l of ithe relevant-and Lpass iloutzthrough slots 2I I] I in ..the Supper and. lower surfaces. :In-

zsidefthe elevon, the. cables-$8 arentraineduover ia ipaiizof sheave-wheels. I 02 wvhich are arrangedson opposite sides .of andispacedverticaIlyIfrom the ahinge 'iaxis Si'l'I.

lttwillbeseen f romthefforegoing that swinging movement:ofttheafiipper:56 relative :to the' wing 1 :responsive to the pressure conditionscbtained for agiven angle of attack is.transmitted to the tab throughthe interconnecting 'linkage causing "theltab to be moved inthesame\direction through :azproportionai angular displacement, "the:ratio an proportionality being governed by the stela'tive --length ofarms 94'to horns =96 and-of arms 91 to horns 99. A hinge moment is thusproduced 'on the elevon tending to resist: movementof -the latterwiththeflipper and to'maintaintheelevon in its-original, undisturbedposition.-If-th'e elevon I O is 'deflected'in one direction bythepilo'hwhilethe-flipper remains stationary, the tab 45 i is defiected in theopposite direction-relativei to the :elevon -:by reason of the "fact'that the oables ea, beingattached :atitheinforward iends to the:stationary arms 9-! and passing on either sideofsthe i'hinge axis 93 ata distance therefrom, act in the same manner as parallel links. l hisitype -"of booster or control tab action 'for the:-purpose ofreducing-the hinge moment on a control: surface is well known in theart; buthas heretotore been obtained 'by connecting the tab horn "afixeii :point on the top or-bottom surface -of the 'wing itself. Thepresent invention, howeverpconibines the advantageous features of boththe' boo'ster tab andthe stabilizing tab, with 1 resulting :improve-'ment in the controllability and stability ioffithe airplane. Thee'iiectiveness of the I:booster' tab action'is determined by theratio-of thelldistance or the axes of sheave wheels I02 from the 'hingeaxis tfito the length of the momentarms of hor-ns "99,-andis increasedby spacing theisheave wheels farther away from the hinge axis fiil.

Another aspect of my inventiontrelaitesto its:

use in conventional airplanes for the purpose of preventing stalls.'This formof theiinvention is illustrated in Figs. '8 and 9 whereinthe-reference numeral IIG indicates an airplane havinga wing I I Iand'fuselage I I2. Carried at the tail end or 1 the fuselage II2 arefixed stabilizers 113, and mounted on the trailing edges of 'thestabilizers are elevators H4. The :elevatorsIIH are-sup- 'portecl'forvertical swinging movement onl'the stabilizers by meansof bearings I tSand' I I Bf-the latter being hollow centered #to permit v passage ofapush rod I I1. Tabs I28 are' hinged to the trailing-edges oftheelevators. A pair of -flipperstfll are disposed in the trailing edgeofthe-wing I-I-I in symmetrical arrangement oneither sneer the fuselageH2 and are pivotally supported forwertical swinging movement. T Rigidlyconnected "to each of the flippers I-I-8-'is-a transverselyextending'rodor torque tube H9 having an upwardly extending arm 121T attachedthereto'withirrthe 58 fuselage. t2 lcconnects the-:arm ..I Zllrtoezatransversely :extendinglarm r122 of: a :bell scrank 123. .sThe bellcrankI23 .isgpivotallyumountednn a verticalihinge pin IZA in the tail ofitheifuselage and 'has anrearwardly extending rarm I2 5 which isslotteclat I26. 5A pin12Ifixed1to'c-the;rodIII is confined :within ..andslides ralongtherslot I25.

The..-outer ends of the link .II I .are connected lto -arms-Iw :of hellcranksilSI. Bell crankstl3l are tpivotally supported on :their:respectivev rud- JdersJ I4 by vertical .pivot pins I32 and hava-lat-:erally 'extending arms I33 which are :connected by link I 34.to hornil35 .on .thetab. I 28.

:The principle :of the :linkage connecting the flippers .l 18 toithewtabs l:28;is seenztozbezsimilar to that: shownin theofirstembodiment, and. the -position of the tabsis governed solely-by the :po-'sition of the-flippers.

When the airplane: is flying. atiathighxangle of attack and the wing ison the verge oflsstalling, the flippers l I 8 swing upwardly'responsivetothe pressure of distribution obtained: for a partially-stalled wing, cauSi-ng the tabs 428 to swing .up- -wardly also. Upward:d'eflection of the tabs 11-28 causes I the elevators to swingdownwardly zrelative to "the stabilizers 11-3, -producingzatdivingmoment in the airplane tending to reduce 1 the angle of attack and-thereby averting the impending stall.

Thepurposeof using two flippers I I8 arranged ion-either side-of thefuselage I2 land rigidly interconnectedby the shaft H9 :is to eliminatethe undesirabletab-controlacticn =which would otherwise be obtainedwith-a single fiippenwhen the flight conditions are unsymmetrical, suchas; for example, when the airplane being -rolle'd. oppositelyacting-moments in' the flippers 118 are thus cancelled out-and-the-nettab-control =action is a functiomof 'the mean angl'e of attack of thewing.

"It isbelieved that a fullunderstanding of the invention will" behadfrom the foregoing,- but it is'to be understood-that" thedrawings-anddescription "are to becon'sidered as illustratively of "andnotrestrictive on-"the broader claims' 'appended hereto, for variouschanges in 4design, structure,- and arrangement may bemade withoutdeparting from the spirit-and scope "df' said claims.

"I'claim:

1. In anaircraftythecombination-of aaving, an elevator pivotallysupported on the aircraft, "a tab hinged to the trailing edge ofthe-elevator, a flipper in thetrailingedge of said wing piv- 'otallymounted for swinging movement ina vertical planeand positioned withrespect to said wing to swing inaccordance with changes in chordwisepressure distribution over said wing; said-flipper beingfree'to trailupwardly athigh angles of "attack, and linkage meansoperatively-connecting said" flipper with said tab to move said tab toangles difiering from the trailing angle thereofysaid'flipper and tabconnecting means passing "through the axis of rotation of said elevatorwhereby 'the'angular relationship of said tab tofsaid elevator isdetermined solely by the positionof 'said flipper and is unaffected "bythe angular position ofthe elevator.

2.*Inan"aircraft, the combination of a wing, an "elevator, means on saidaircraft including 'a holl'ow'bearingfor supporting said elevator "forpivotal'movement, a tab hinged to the trailing edgeof said elevator, 'afreely trailingfiipper gpivotally mounted in the trailing edge 'of saidwing;- and linkage" means connecting said fli-pper with said tab wherebythe position of the tab relative to the elevator is determined by theposition of the flipper relative to the wing, said linkage meansincluding a push-pull rod slidably extending through said hollowbearing, and means connected with said tab for changing the position ofthe tab responsive to sliding movement of said push-pull rod throughsaid hollow bearing.

3. In an aircraft, the combination of a wing, a control surface arrangedin the trailing edge of said wing, means supporting said control surfaceon said wing for vertical swinging movement including a hollow bearing,a tab hinged to the trailing edge of said control surface, a flipperdisposed alongside said control surface and pivotally supported on saidwing for vertical swinging movement, a first bell crank pivoted on saidwing for rotation about a substantially vertical axis, a link connectingone arm of said bell crank to said flipper at a point spaced verticallyfrom its axis of rotation, a push-pull rod having one end connected tothe other arm of said bell crank and extending slidably through saidhollow bearing into the interior of said control surface, a second bellcrank arranged within the interior of said control surface and havingone arm thereof connected to the other end of said push-pull rod, andlink means connecting the other arm of said last-named bell crank tosaid tab for operating the same.

4. In an aircraft, the combination of a wing, a control surface arrangedin and pivotally supported on the trailing edge of said wing, a tabhinged to the trailing edge of said control surface, a flipper disposedalongside said control surface and pivotally supported at the trailingedge of said wing, a member rotatably supported on said wing, meansconnecting said member with said flipper whereby movement of the flippercauses said member to rotate, and force transmitting means connectingsaid member with said tab and passing the pivot axis of said controlsurface at a distance therefrom whereby movement of the control surfacerelative to the wing causes said tab to be deflected in a direction andamount to produce a hinge moment assisting such movement, while movementof the flipper relative to the wing in a given direction causes said tabto be deflected in a direction and amount to produce a hinge moment onthe control surface acting in the opposite direction.

5. In an aircraft, the combination of a wing, a control surfacepivotally supported at the trailing edge of said wing and connected tobe moved by a pilot of said aircraft, a flipper, pivotally supported atthe trailing edge of said wing and free to trail in accordance with thechordwise pressure distribution over said' wing ahead of said flipper, atab hinged to said control surface at the trailing edge thereof along ahinge line parallel to that of said flipper, and means connecting saidflipper to said tab to move said tab to angles differing from thetrailing angle thereof when said control surface or said flipper ismoved away from neutral position to apply an additional hinge moment tosaid control surface.

6. Apparatus in accordance with claim wherein said connection is made tocreate said additional hinge moment in opposition to a hinge momentcreated on said control surface by virtue of a change in angle of attackof said wing.

'7. Apparatus in accordance with claim 5 wherein said connection is madeto create said trail upwardly at high angles of attack of saidwing.

8. Apparatus in accordance with claim 5 wherein said control surface andsaid flipper are positioned closely adjacent at the trailing edge ofsaid wing to have closely related trailingtendencies.

9. In an aircraft, the combination of a win a pitch control surfacehinged to said aircraft and connected to be swung vertically by thepilot of said aircraft to changethe angle of attack of said wing, aflipper located in the trailing edge of and forming a part of said wing,and pivotally hinged thereto on a horizontal axis with respect to saidaircraft when in level position for free vertical swinging movementswhen acted upon by changes in the chordwise pressure distribution overthe wing due to changes in the angle of attack, said flipper, because ofsaid pressure changes, having a characteristic tendency at low ormoderate angles of attack to deflect upwardly with respect to the wingin relatively small angular increments for a iven increment in angle ofattack, and as the wing approaches the stalling angle, to deflectupwardly in considerably larger angular increments, a tab hinged to saidcontrol surface along a line paral lel to that of said control surfaceand movable with respect to said surface to produce a hinge momentthereon, and means connecting said flipper to said tab to move said tabto angles producing hinge moments on said control surface substantiallyproportional to the angular deflection of said flipper and tending tomove said control surface in a direction to reduce the angle of attackof said wing.

10. In an aircraft, the combination of a wing, a pivotally supportedpitch control surface having a movable element thereon capable ofproducing an aerodynamic force to create a hinge moment on the controlsurface, said control surface being connected to be moved by the pilotof said aircraft to change the angle of attack of said wing, a flipperlocated in the trailing edge of and forming a part of said wing, andpivotally hinged thereto on a horizontal axis with respect to saidaircraft when in level position for free vertical swinging movementswhen acted on by changes in the chordwise pressure distribution over thewing due to changes in the angle of attack, said control surface andsaid flipper having a characteristic tendency at low or moderate anglesof attack to deflect upwardly with respect to the wing in relativelysmall angular increments for a given increment in angle of attack, andas the wing approaches the stalling angle, to deflect upwardly inconsiderably larger angular increments, and means connecting saidflipper to said element to move said element with respect to saidcontrol surface in a direction and by an amount to produce a hingemoment on said control surface of progressively increasing magnitude asthe wing approaches the stalling angle.

11. In an aircraft, the combination of a wing, a pitch control surfacepivotally supported for vertical movement on said wing and forming apart of the trailing edge thereof and connected to be moved by the pilotof said aircraft, a tab hinged to the trailing edge of said controlsurzgsmssz orrr byi changes in the chordt'vise pressure dis-i gtribution over the wing due to changes in the angl'eiof attack, saidcontrol surface: and said flipperr having: a characteristic: tendency atlow or moderate angiesrof attack to deflect upwardly witlir'espect totheiwiirgdnerelatively small ango? lar increments for a given incrementint angle 01$:attack and as thewing approaches the stallangle; todeflect upwardly" in considerably larger 'angula'r 'increments, and'means connectin'g sa'id fiipper to' said t'ab to move the tab indirection and tea-degree creating a hinge moment on-= said controlsurface substantially counter=ba1ancing the tendency of said controlsurface t'o' -trail upwardly with saidzflipperc Bi SEARS.

1 2 REFERENGES OI'DED' The" following references; are of record .in..tl'i'e'. fife: of this -patent UNITED. 'STATES PATENTS Number Name Date15464,?841 Tarbox- Aug; 1451923 573,109 Zotte1 Feb. 1651926 1,659,960Rohrbach 4Fb.'- 21; 1928 1,851,797 Alineida Mar; 29; 1932 1,935,824Upson" Nov. 21'; 1933" 2,057,877 Bragunier och-20;- 1936 2156;994'Lachmann May 2 f 1939 2,251,304. Miller Spt2--2;' 194-1 2,368,059 WhiteJan-s 23,-- 1 945 FOREIGN PATENTS? Number Country Date;

691,115-.- Germany May 17; 1940- 694506 Germany= July 29; 1940:

23,309 France 1-. June-6;, 1921' (Addition: to) No. 400,578)

